The invention relates to a method for reducing interference in radio transmitters, which have feedback from the antenna end to the baseband side. The invention further relates to a transmitter applying the said method.
In all radio systems utilizing multiple carrier wave frequencies it is important that the signals in the different frequency division channels not interfere with each other. Non-interference requires that transmission power levels are kept low enough and the spectrum of the transmission signal is confined as accurately as possible to the frequency band allocated to it. Spreading of the spectrum is caused by the modulation required by radio transmission and in particular by nonlinearity in the transmitter units. The latter results in various extra spurious spectra which may be located in bands corresponding to the other frequency division channels. In practice, there is always some nonlinearity in mixers and amplifiers, in principle the more, the higher the level of the signal. So, in this respect the power amplifier in a transmitter may prove to be a problematic component.
Filters have been conventionally used to limit the spectrum of a signal. In practice, however, the linearity of the transmitter must also be taken into account, in addition to filtering. One way of improving the linearity is to arrange for feedback from the antenna feed point to the baseband side so that a change is made in the signal going to the modulator, which change is equal to the distortion but opposite to it. This type of correction is represented by the so-called Cartesian loop shown in FIG. 1. The figure shows a simplified block diagram of a transmitter comprising, connected in series, a modulator 103, a first level control unit 110, radio-frequency power amplifier 120, directional coupler 130, and an antenna 140. The modulator is of the quadrature type: it has got two branches, I (in-phase) and Q (quadrature phase), which both include an analog multiplier that shifts the signal spectrum into the radio-frequency range. Carrier waves of the same frequency are brought to the analog multipliers and a phase difference of 90 degrees, necessary for the operation of the modulator, is produced between the carrier waves in a block of its own. The carrier wave comes from a local oscillator 171. The signals output by the analog multipliers are summed up, producing the modulator output signal SM.
The level control units 110 and 150 may be variable attenuators or variable amplifiers. The attenuation or gain may be adjusted in a continuous or stepwise manner. In this description and in the claims we will only use the term attenuation as regards the level control unit. Attenuation may also be negative, which means (positive) gain.
The feedback branch of the Cartesian loop begins at the side port p1 of the said directional coupler 130, to which port a small fraction of the energy fed towards the antenna by the power amplifier 120 is transferred. The signal SFB from the port p1 is taken to the second level control unit 150 and from there to a quadrature demodulator 160. The carrier waves used by the demodulator are synchronized with the carrier waves of the modulator 103, so the demodulator produces signals shaped like the input signals of the modulator. In FIG. 1, the output signal of the upper or I′ branch is marked sI, and the output signal of the lower or Q′ branch is marked sQ′.
For modulation, the data signal to be transmitted is divided into two signals sI1 and sQ1, which are the input signals of the structure shown in FIG. 1. They would be taken straight to the modulator were linearization not used. In FIG. 1, the difference of the I branch input signal sI1 and the corresponding signal sI′ generated by the demodulator is produced in a differential amplifier 101 the output signal sI2 of which is then taken to the analog multiplier of the I branch of the modulator 103, to the baseband input thereof. Similarly, the difference of the Q branch input signal sQ1 and the corresponding signal sQ′ generated by the demodulator is produced in a differential amplifier 102 the output signal sQ2 of which is then taken to the baseband input of the Q branch of the modulator 103. This closes the Cartesian loop. The feedback provides information on the distortion caused primarily by the power amplifier, and the loop attempts to minimize this distortion, i.e. linearize the transmission path. Linearization is based on the fact that when the loop gain is high enough and the feedback fast enough, signal sI′ is forced into the shape of signal sI1 and, correspondingly, signal sQ′ is forced into the shape of signal sQ1. Thus the signal fed to the antenna, from which signals sI′ and sQ′ have been developed, corresponds to the undistorted baseband signals and therefore has a purer spectrum than if no feedback were used. For the waveforms of signals sI1 and sI′ and, correspondingly, the waveforms of signals sQ1 and sQ′ to be coincident in time, the delay in the signal caused by the transmission path to the antenna must be accounted for. To this end, the carrier wave from the local oscillator 171 is taken to the demodulator direct, but to the modulator via a phase shifter 172. A value is determined for the phase difference Δφ produced by the phase shifter such that the above-mentioned delay is cancelled from the feedback point of view. The phase shifter could as well be located in the branch leading to the demodulator, in which case the phase difference Δφ should be opposite to the one depicted in FIG. 1.
FIG. 1 further shows a processor 180 to set the attenuations of the first and second level control units. This kind of a structure is known from patent publication EP 0 638 994. The idea therein, in addition to linearization, is to set the output level of the transmitter suitable while at the same time keeping the loop gain, which is important for the functioning of the loop, unchanged. This is realized by changing the transmitter output level, or the level of signal sP, by driving the second level control unit 150. The said level depends directly on the setting of the second level control unit since the levels of signals sI′ and sQ′ are in practice constant. The first level control unit 110 is simultaneously driven in the opposite direction so that the combined attenuation of the level control units stays constant. The drawback of the structure is that the noise caused by the modulator 103 may reach a detrimental level. This results from the fact that component tolerances cause variation in the levels of the modulator input signals sI2 and sQ2 in individual devices. The input signal levels may be relatively near the noise level, whereby naturally the signal-to-noise ratio of the modulator output signal and the output signal of the whole transmitter is relatively poor.